The elderly are the fastest growing subset of the US population. Age-associated increases in chronic disease and disability represent a significant financial burden on the healthcare system. Glomerular filtration rate (GFR) declines with age and low GFR defines chronic kidney disease (CKD). Many older adults have CKD, which is an important risk factor for adverse outcomes. Accurate assessment of GFR is central to clinical decision-making, assessment of the public health burden, and investigation into the epidemiology of CKD in older adults. However, there is active debate about what constitutes "normal" level of GFR in older adults and the best method to assess it. Creatinine based GFR estimates may not be accurate in older adults as malnutrition and loss of muscle mass, common with age and disease, affect serum creatinine independent of GFR. Cystatin C is an altnernative filtration marker not influenced by diet and muscle mass. The overall goal of this research is to explore the relationship of kidney function and aging. Three aims are proposed: (1) Develop and validate GFR estimating eqations from cystatin C in the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) database (N>65 years= 1306). (2) Examine relationships among estimated GFR based on cystatin C, aging, nutrition, body composition, and mortality in the National Health and Nutrition Examination Survey (NHANES) (N>60 years= 7525). (3) Conduct a pilot study to examine the level of measured GFR in the general population of older adults (N=100) for subsequent implementation of ancillary studies to measure GFR in ongoing prospective cohort studies to provide information on normal levels of GFR across age and populations. A mentored, patient oriented Career Development Award is proposed to provide the candidate with rigorous methodological training and experience in clinical research related to kidney function and aging. The candidate proposes research projects and a career plan that includes didactic courses, expert advising, and mentored research to enhance the candidate's skills in complex statistical analyses of population based and longitudinal data, primary data collection and human subjects research, and to expand the candidate's kidney function and clinical geriatrics knowledge base. These projects and career development plan are consistent with the recognition by the Center for Disease Control of aging and chronic kidney disease as major public health problems.